Rotors utilized in ultrahigh speed centrifuges are driven at speeds from about 20,000 rpm up to speeds approaching and exceeding 100,000 rpm's. The balance of such rotors when driven at such speed is obviously very critical. Any imbalance can cause the rotor to become detached from the drive spindle resulting in damage to the rotor and the centrifuge. The lighter the rotor the more critical the balance since the heavier the rotor the greater the downward force and the less tendency for the rotor to "jump-off" the drive spindle because of imbalance. Moreover, any imbalance may repeatedly shut down the centrifuge if it is equipped with an imbalance detection system.
In the past fixed angle and vertical tube ultracentrifuge rotors have been made of isotropic material such as aluminum or titanium. The rotor is cast in a billet and then carefully machined to form the rotor and the test tube cavities are drilled therein. In such rotors it has been the practice to balance the rotor in the lower plane by removing material from the lower surface of the rotor by milling, sanding, machining or filing and in the upper plane to remove material from the outer upper periphery thereof in like manner. Since the rotor typically sets atop its drive spindle, balancing in the upper plane of the rotor is more critical.
High speed hybrid rotors have recently been introduced. Such rotors include a rotor core or body of isotropic material with a reinforcing ring around the outer periphery thereof in the form of a filament wound graphite fiber and epoxy resin ring. In one method of construction the isotropic core is cryogenically cooled to a very low temperature to shrink the core, the ring place around the core body and the combination allowed to return to normal temperature. As the core body returns to room temperature, the core expands against the reinforcing ring thereby prestressing the ring and core body.
Balancing of hybrid type rotors by the conventional method is undesirable in that removal of material from the outer diameter of the core body disturbs the tight tolerances necessary to create the pressure and interference fit between the core body and the reinforcing ring. Further, balancing of the core body prior to assembly with the reinforcing ring does not insure balance of the hybrid rotor combination. Any attempt to balance the rotor in the upper plane by removal of material from the reinforcing ring breaks the fiber filaments damaging the structural integrity of the ring which leads to a reduction in its strength defeating the purpose of the hybrid type rotor.